1. Field of the Invention
The present invention relates to an electroacoustic transducer for transforming electronic signals into acoustic vibrations.
2. Description of the Prior Art
The conventional electroacoustic transducer has been constructed as illustrated in FIG. 20 and 21. A resonant chamber 102 is formed in front of a diaphragm 100, and a sound ejector 106 is incorporated with an outer case 104 whereby the resonant chamber 102 is enclosed. The sound ejector 106 is provided with a sound ejecting hole 108 communicating the resonant chamber 102 with the outside air. The diaphragm 100 is made of a magnetic substance and supported by a cylindrical magnet 110 served as a supporting means fixed on the back thereof. Furthermore, a magnetic driver 120 is placed on the central part of the backside of the diaphragm 100. The magnetic driver 120 transforms electronic signals into magnetic vibrations to produce mechanical vibrations in the diaphragm 100, and is mounted on a base 123 that forms a closed magnetic circuit with an iron core 122 and the magnet 110. A coil 124 is wound around the iron core 122. Furthermore, the terminals of the coil 124 are individually connected to a lead terminal 126 and 128 which are isolated from and mounted upright on the base 123. Input electronic signals are applied between the lead terminal 126 and 128.
FIG. 22 shows a sound pressure vs. frequency response characteristics (overall) of the electroacoustic transducer shown in FIG. 20, FIG. 23 shows a current vs. frequency response characteristics (overall) thereof, FIG. 24 shows a sound pressure vs. frequency response characteristics near the frequency where the response of the sound pressure becomes maximum, and FIG. 25 shows a current vs. frequency response characteristics near the frequency where the response of the sound pressure becomes maximum. In FIG. 24, there is not any irregularity on the waveform representing a chattering at D near 800 Hz. Furthermore, FIG. 26 shows a sound pressure vs. frequency response characteristics (overall) of the electroacoustic transducer shown in FIG. 21, and FIG. 27 shows a current vs. frequency response characteristics (overall) thereof. In FIG. 26, the sound pressure characteristics around E show a comparably flat response.
In the case of the electroacoustic transducer shown in FIG. 20, the sound ejecting hole 108 is placed on the central axis O, however, the position can be shifted depending on the functional requirements for ejecting sounds. In recent years, in electronics such as a portable telephone which employs such an electroacoustic transducer, the sound ejecting hole 108 is often subject to change of the position from which sounds are ejected in compliance with various requirements for miniaturization, flatness, and the like.
Accordingly, an electroacoustic transducer in which the sound ejecting hole 108 is formed at an irregular position has been proposed in order to comply with such requirements, The electroacoustic transducer shown in FIG. 28, 29 has the sound ejecting hole 108 formed on the ceiling close to the side wall of the outer case 104. The electroacoustic transducer shown in FIG. 30 has the sound ejecting hole 108 formed on the side wall of the outer case 104. The foregoing positions where the sound ejecting holes 108 are formed are shown as an example, and the sound ejecting hole 108 can also be formed on the corner of the outer case 104.
Incidentally, in the electroacoustic transducer in which the sound ejecting hole 108 is formed at a position off to the central axis O of the diaphragm 100, the resonance frequency of the resonant chamber 102 can be tuned by changing the diameter and length of the sound ejecting hole 108, even if the sound ejecting hole 108 is placed off to the central axis O of the diaphragm 100. However, the relation between the vibration of the diaphragm 100 and the sound ejecting hole 108 becomes weak. In consequence, the air damping effect by the sound ejecting hole 108 weakens, and such an electroacoustic transducer is apt to assume acoustic characteristics different from that of the electroacoustic transducer with the sound ejecting hole 108 around the central axis O of the diaphragm 100. The electroacoustic transducer is likely to be required for a higher power, wider frequency range, and higher sound quality as well as miniaturization. In order to comply with such requirements, the acoustic load by the resonant chamber 102 has generally been utilized as an air damping factor when the vibration system vibrates in a higher amplitude.
In the electroacoustic transducer shown in FIG. 29 and 30, increasing the vibration amplitude will make a sharp sound pressure vs. frequency response characteristics, or cause chatterings around the peripheral support of the diaphragm 100. FIG. 31 shows a sound pressure vs. frequency response characteristics (overall) of the electroacoustic transducer shown in FIG. 29, FIG. 32 shows a current vs. frequency response characteristics (overall) thereof. In FIG. 31, the characteristics around F shows a sharp response of the sound pressure. Furthermore, FIG. 33 shows a sound pressure vs. frequency response characteristics (overall) of the electroacoustic transducer shown in FIG. 30, FIG. 34 shows a current vs. frequency response characteristics thereof, FIG. 35 shows a sound pressure vs. frequency response characteristics near the frequency where the sound pressure becomes maximum, and FIG. 36 shows a current vs. frequency characteristics near the frequency where the sound pressure becomes maximum. FIG. 35 shows that chatterings are produced around the periphery of the diaphragm 100 at G near 800 Hz, namely, at the maximum amplitude. In order to avoid such a phenomenon, the requirements for the diaphragm 100 cannot be ignored. It is undeniable that thinning the thickness of the diaphragm 100 in comparison with the diameter thereof is apt to increase higher harmonics owing to chatterings and divided vibrations.
Such a phenomenon and the countermeasure thereof have been disclosed, for example, in JP-U-56-52719, in which a countermeasure to increase the acoustic impedance inside a resonant chamber is clarified. Although such a countermeasure can be considered to be effective in damping the foregoing phenomenon, it is possible to decrease the volume of a resonant chamber and thereby to weaken the resonant effect thereof.